(1) Field of the Invention
This invention relates to systems and methods for coating electrically conductive substrates, and more particularly to multi-layer systems and methods for coating electrically conductive substrates.
(2) Description of the Related Art
Throughout this patent application, the word “base” to qualify an alloy means that the alloy contains at least 50%, by weight, of the qualified element, for example, “copper-base” means more than 50%, by weight, of copper. Copper and copper-base alloys (hereinafter generally referred to as “copper”) are commonly used in electrical and electronic industries for connectors, electrical harnesses, printed circuit boards, ball grid arrays, leadframes, multichip modules, and the like. While copper provides excellent electrical conductivity, it is known to easily oxidize and tarnish when exposed to elevated temperatures, moisture, or chemicals. Oxidation and tarnishing of copper generally result in high electrical contact resistance, thereby reducing performance of electrical devices. In addition, oxidation and tarnishing of copper reduce the wettability of solder, and generally makes soldering problematic.
One method to reduce oxidation and tarnishing of copper is to apply a tin or tin-base alloy coating (hereinafter generally referred to as “tin”) onto a copper substrate. The tin coating acts as a barrier to prevent or reduce oxidation, and thereby maintain the electrical performance of the substrate. However, there are many problems associated with using tin as a coating layer on a conductive substrate. Rapidly at elevated temperatures, and more gradually at room temperature (nominally 25° C.), the tin coating interdiffuses with the copper substrate to form copper-tin intermetallic compounds (IMC's). These IMC's reduce the thickness of the tin coating layer and cause an increase in contact resistance and degradation of solderability.
Exemplary thermal excursions include 250° C. for a few seconds during wire bonding or encapsulation in a polymer, 300° C. for a few seconds during reflow and 150° C. for from 8 to 168 hours for a controlled reduction of tin thickness to reduce friction.
One approach taken to reduce the effect of IMC formation and maintain a low contact resistance is to use a thicker tin coating; however, this approach not only increases the cost of the parts but also causes some functional problems. Where the tin coating is used on an electrical connector, a thicker coating of soft tin increases friction leading to an increased insertion force, making plugging and unplugging the connector physically difficult. For electronic devices, a thicker tin or tin alloy coating is also undesirable since the trend is to make electronic devices thinner and smaller. Moreover, where the tin coating is used on leads of an electronic device, a thick tin coating can cause problems in the coplanarity and fine line definition of the leads.
Another approach taken to reduce the effect of IMC formation is to use a transition barrier layer between the copper substrate and the tin coating to inhibit the growth of IMC. For example, U.S. Pat. No. 4,441,118 reports low IMC growth rates using a copper-nickel alloy substrate with 15-30% nickel.
In another example, a publication by P. J. Kay and C. A. Mackay, in Transactions of the Institute of Metal Finishing, Volume 51, 1979, at page 169, discusses the use of various metals as transition barrier layers. In one example, this publication describes a silver barrier layer having a thickness of 1 micrometer. However, this example was shown to be undesirable because the silver transition barrier layer resulted in no substantial reduction in the diffusion rate between copper and tin. U.S. Pat. No. 4,756,467 to Schatzberg discloses a solderable connector having a copper substrate, a thin layer of silver, a silver-tin alloy layer and an outermost tin layer. The silver-tin alloy layer is formed by a diffusion anneal. Japanese Patent Number 2670348 (publication number 02-301573) to Furukawa Electric Co. Ltd. discloses a copper substrate coated with a barrier layer that is nickel or cobalt, followed by silver layer followed by a melt-solidified layer of tin or tin alloy.
Commonly owned U.S. patent application Ser. No. 10/930,316, that was filed on Aug. 31, 2004 as a continuation of U.S. patent application Ser. No. 09/657,794 discloses a thin anti-tarnish layer disposed between a copper substrate and a tin coating layer. Among the metals disclosed as anti-tarnish layers are zinc, chromium, indium, phosphorous, manganese, boron, thallium, calcium, silver, gold, platinum, palladium and combinations and alloys thereof.
Other barrier layers are disclosed in commonly owned U.S. Pat. No. 5,780,172, to Fister et al., and commonly owned U.S. Pat. No. 5,916,695 to Fister et al. U.S. Pat. Nos. 4,756,467 and 5,916,695 as well as U.S. patent application Ser. No. 10/930,316 are incorporated by reference in their entireties herein.
Another problem associated with the use of tin as a coating layer for a conductive substrate is that tin is susceptible to fretting corrosion. Fretting corrosion is the oxidation of contact surfaces that results from relative motion (fretting) between two mating contact surfaces. The oxidation caused by fretting can result in an unacceptable increase in contact resistance. Certain metals, such as silver, are known to have excellent resistance to fretting corrosion. However, silver tends to tarnish in the atmosphere due to the presence of sulfur dioxide, which causes silver sulfide to form on the surface of the silver. The tarnish is aesthetically unacceptable and could degrade the functional properties of the electrical contact.
Yet another problem associated with the use of a tin coating layer, as well as other coating layers such as zinc, indium, antimony, or cadmium, on a conductive substrate is that tin is susceptible to whiskering. Whiskering occurs as the tin ages and stresses in the tin or at the tin/IMC interface begin to build. Whiskering also occurs due to internal stress resulting from the plating process. To relax the stress, single crystals of tin nucleate from the surfaces like whiskers. Each whisker continues to grow until the internal stresses subside. Whiskering can cause many different problems, including shorting of adjacent electrical contact surfaces. Alloying the tin coating with a small amount of lead (Pb) is a common means of reducing whisker growth. However, because of health and environmental reasons, many industries are striving to reduce or eliminate the use of lead.
Thus, there is a need to develop a coating system that would be able to maintain a low contact resistance and good solderability after fretting and thermal exposure, combined with one or all of the additional attributes of lower coefficient of friction, and reduced whisker growth.